Hi all,

I just encountered something that I have not seen before and I am
interested to see if others have encountered this. I have a PCB with
several 78xx series regulators on it which were working fine. I then
made some modifications which meant that the loads which were
connected to the 7815 and 7812 regulators were not needed, so I
disconnected them. I then began to have problems with a periodic
signal occurring on the main 24V rail. I eventually discovered that
both the 7815 and 7812 were oscillating and because they have large
output capacitors, this was drawing significant AC current from the
24V rail.

I searched two 78xx series datasheets, as well as a quick Google
search, and found no mention of a minimum load for 78xx series
regulators, although I do notice that in all over the specifications,
they never mention conditions with an output current of less than 5mA.

Anyone seen this problem before? It makes sense given the design of
the regulator but I am surprised that I've never seen it mentioned
before since other kinds of regulators, such as switchers, often have
a listed minimum load current.

Sean
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